The present invention relates to a process for the production of a subcooled liquefied natural gas stream from a natural gas feed stream, the process being intended to form liquefied natural gas (LNG) subcooled to a temperature below −120° C.
Such a process is to be carried out especially in the vicinity of natural gas production sites in order to convert a natural gas feed of large volume into a liquefied natural gas of reduced volume. The LNG can especially be loaded into methane tankers in order to be transported by sea to consumption centres.
The process is to be carried out either in new natural gas liquefaction units or in existing units.
For the liquefaction of a natural gas feed, a process comprising a plurality of refrigeration cycles using refrigerant fluids formed by C2+ hydrocarbons is frequently used.
An example of such a process, denoted by the acronym “C3/MR”, uses two refrigeration cycles, a first cycle that uses propane and a second cycle in which a mixture of refrigerants composed of nitrogen, methane, ethane and propane is used.
The refrigerants circulating in these cycles are at least partly liquefied after compression in gas form, for example by undergoing static expansion in an expansion valve.
The liquid so obtained is placed in a heat exchange relationship with the feed at different temperature levels. The liquid refrigerant evaporates by heat exchange with the feed, thus supplying frigories for the cooling, liquefaction and subcooling of the feed. These refrigeration cycles can be adapted thermodynamically to the feed in order to obtain very high efficiencies.
However, such processes require refrigerants to be used and stored in liquid form, both inside and outside the liquefaction unit. Accordingly, hydrocarbons such as propane and ethane must be produced or supplied in liquid form and stored at least temporarily. This requires particular precautions in terms of safety and an infrastructure adapted to avoid accidents.
Such processes are therefore not suitable for environments having a small available space and/or safety constraints, such as especially the floating units for the recovery, storage and treatment of hydrocarbons designated by the acronym “FPSO” or platforms located at sea directly above submarine natural gas fields.
Accordingly, it is necessary in the latter two cases to convey the natural gas to a liquefaction unit located on land, which is not very economical and requires the laying of a submarine pipeline.
In order to remedy that problem, there is known from U.S. Pat. No. 5,768,912 a process of the above-mentioned type which employs, for precooling and liquefying the natural gas, a first cycle using a conventional mixed refrigerant followed by a second complex cycle comprising three dynamic gas expansion turbines.
In the second cycle, the refrigerant fluid is formed substantially of nitrogen, which remains gaseous for the entire time for which it circulates in the cycle.
Such a process therefore makes it possible to overcome some of the difficulties associated with the transport, storage and use of dangerous liquid refrigerant fluids formed by C2+ hydrocarbons. In addition, it is relatively insensitive to the movements of the unit in which it is carried out, especially when that unit is on board a floating system.
Nevertheless, that process exhibits a low thermodynamic efficiency, which renders its use expensive in terms of energy.